The structural organization of the nucleolus, especially the DNA-containing structure, was examined in Vicia faba root-tip cells exposed to hypoxia. Hypoxic conditions were attained by boiling distillled water and immediately cooling down to 23ﾟC.The nucleoli suffered stepwise structural alterations with exposure time as follows ; (1) nucleolar segregation, (2) nucleolar dispersion, (3) appearance of argyrophilic stands and (4) complete disappearance. Electron microscopy showed that the fibrillar centers (FCs) gradually develop with appearance of large particles (35 nm granules) inside and concomitantly with these phenomenon, chromatin and endomenbrane began to collapse. When immunogold labeling with an anti-DNA antibody was applied, the dense fibrillar component (DFC) was moderately labeled in cells of an actively growing root, whereas under hypoxia the label on the DFC gradually decreased with time and chromatin-like masses in the Fcs were most heavily labeled. This suggest that rDNA engaged in transcription retired from the DFC into the FCs under hypoxia.Thick and thin nucleolonema were seen in the nucleolus of Raphanus sativus. In meristematic cells, the two types of nucleoli, those with mainly thick nucleolonema and with mostly thin nucleolonema, were seen while the nucleoli with thin nucleolonema wes most frequently found in the differentiated cells. Silver staining was applied to thin sections of the specimen embedded in Spurrs resin. As the result, three types of nucleolonema segments were appreciated ; (1) stack of short fragments, (2) two rays of round-shape segments and (3) ring-shape segments. All these segments were arranged in 450 nm width. The appearance of these three types of nucleolonema segments is well elucidated by an idea that the nucleolonema filament coils up to form 450 nm thick nucleolonema. This coiling of the nucleolonema may reflect chromosome solenoid. This coil probably begin to collapse as cells differentiate.